B. Parry

ASE suppression in a high energy Titanium sapphire amplifier

The energy required to generate ultrashort pulses with petawatt peak power from a Ti:sapphire laser system is a few tens of joules. To achieve this, the final amplifier must have a gain region of around 5 cm diameter that is uniformly pumped at high fluence. The high level of amplified spontaneous emission (ASE) in such an amplifier will seriously degrade its performance unless care is taken to minimise the transverse gain and the internal reflections from the crystal edges. In developing the amplifiers for the Astra Gemini laser system, we have combined the techniques of beam homogenisation and double-pass pumping of a lightly-doped crystal with a new index-matched absorber liquid. Our results demonstrate that this combined approach successfully overcomes the problem of gain depletion by ASE in a high-energy Ti:sapphire amplifier.

Investigation of GeV-scale electron acceleration in a gas-filled capillary discharge waveguide

The generation of GeV-scale electron beams in a gas-filled capillary discharge waveguide with good reproducibility is discussed. Beams of electrons with energies above 900MeV, and with root-mean-square divergences of 3.5mrad, are observed for a plasma density of 2.2◊10 18 cm 3 and a peak input laser power of 55TW. The variation of the maximum electron energy with the plasma density is measured and found to agree well with simple models. Injection and acceleration of electrons at the to date lowest plasma density of 3.2◊10 17 cm 3 are reported. The energy spectra of the generated electron beams exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum are described.

Investigation of the role of plasma channels as waveguides for laser-wakefield accelerators

The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.

Comparative study on the temporal contrast of femtosecond mode-locked laser oscillators

We have investigated the temporal intensity contrast characteristics from a broad range of mode-locked short-pulse oscillators used for seeding high-power terawatt and petawatt-class laser systems. Saturable absorber (SESAM), Kerr lens (KLM), nonlinear polarization evolution (NPE) in optical fibers and synchronously pumped optical parametric oscillator (OPO) mode-locked sources have been measured using a third-order autocorrelator with up to 1010 dynamic range. We restricted the temporal characterization to features <30  ps about the laser pulse that reflect fundamental mode-locking processes. We find additional nonlinear terms and residual higher-order dispersion limits the performance of KLM and NPE sources up to the 105 contrast level, while >108 contrast was observed from the SESAM and OPO laser pulse trains.

Commissioning the Astra Gemini petawatt Ti:sapphire laser system

Astra Gemini is a dual-beam petawatt Ti:sapphire laser at the Rutherford Appleton Laboratory in the U.K. We report measurements characterising the laser beam quality, pulse energy, and pulse duration of the first beam line.

Experimental investigation of cross-polarized wave generated with focused spatially dispersed beam

The nonlinear process of cross polarized wave (XPW) generation, which has been broadly used for temporal contrast enhancement and spectral broadening [1, 2], continue to attract interest in different application schemes. A feature of space-time focusing (STF), which occurs at the focus of spatially dispersed beam, is to deliver highly localized intensity at the focus without incurring high non-linear losses during long distance propagation. A number of nonlinear optical processes have been used to study and to utilize STF: two-photon fluorescence [3], harmonic generation [4], plasma formation [5] and refractive index modification in solids [6]. In this paper XPW generation in BaFz has been studied at the SST conditions of a compressed femtosecond pulse.

Electron acceleration driven in plasma channels at the Astra-Gemini laser facility

The generation of GeV-scale electron beams in the plasma channel formed in a gas-filled capillary discharge waveguide is investigated. Electron beams with energies above 900 MeV and with root-mean-square divergence of 3.5 mrad are observed for plasma densities of 2.15 × 1018 cm−3 and a peak input laser power of only 55 TW. The variation of the electron energy with the plasma density is measured and found to exhibit a maximum at plasma densities for which the dephasing length approximately matches the length of the plasma channel. Injection and acceleration of electrons at the relatively low plasma density of 3.2 × 1017 cm−3 is observed. The energy spectra of the generated electron beams are shown to exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting for the effects on the measured energy spectrum of off-axis electron beam propagation are investigated.

Dual beam operation of the Astra-Gemini high power laser and upgrades to the Ti:Sapphire amplifiers

Special considerations of operating the dual beam facility Gemini are presented. Amplifiers in Astra have been upgraded to improve the contrast, stability and performance with wedged Ti:Sapphire crystals and spatial filtering.

Investigation of Contrast for Astra-Gemini laser

We report the recent progress in contrast compaign for Astra-Gemini, showing that the stretcher grating as the main source of the coherent contrast. Improvement was achieved by new gratings, and further enhanced by using double plasma mirrors.

Contrast Enhancement for Astra-Gemini Laser

We report on the contrast enhancement for Astra-Gemini laser following identifying the major source of coherent contrast, in conjunction with plasma mirrors. Replica prepulses were suppressed or eliminated by employing the wedged optics.